A wireless communication device in a communication system communicates directly or indirectly with other wireless communication devices. For direct/point-to-point communications, the participating wireless communication devices tune their receivers and transmitters to the same channel(s) and communicate over those channels. For indirect wireless communications, each wireless communication device communicates directly with an associated base station and/or access point via an assigned channel.
Each wireless communication device participating in wireless communications includes a built-in radio transceiver (i.e., transmitter and receiver) or is coupled to an associated radio transceiver. Typically, the transmitter includes one antenna for transmitting radiofrequency (RF) signals, which are received by one or more antennas of the receiver. When the receiver includes two or more antennas, the receiver selects one of antennas to receive the incoming RF signals. This type of wireless communication between the transmitter and receiver is known as a single-output-single-input (SISO) communication.
Generally speaking, transmission systems compliant with the IEEE 802.11a and 802.11g or “802.11a/g” as well as the 802.11n standards achieve their high data transmission rates using Orthogonal Frequency Division Modulation (OFDM) encoded symbols mapped up to a 64 quadrature amplitude modulation (QAM) multi-carrier constellation. In a general sense, the use of OFDM divides the overall system bandwidth into a number of frequency sub-bands or channels, with each frequency sub-band being associated with a respective sub-carrier upon which data may be modulated. Thus, each frequency sub-band of the OFDM system may be viewed as an independent transmission channel within which to send data, thereby increasing the overall throughput or transmission rate of the communication system. Similarly, multi-code spread spectrum system comprised of perfectly orthogonal high-speed chaos spreading codes transporting independent modulated data can be used to increase its overall throughput or transmission rate of the SISO system. The high-speed “spreading signals” belong to the class of signals referred to as Pseudo Noise (PN) or pseudo-random signal. This class of signals possesses good autocorrelation and cross-correlation properties such that different PN sequences are nearly orthogonal to one other. The autocorrelation and cross-correlation properties of these PN sequences allow the original information bearing signal to be spread at the transmitter.
Transmitters used in the wireless communication systems that are compliant with the aforementioned 802.11a/802.11g/802.11n standards as well as other standards such as the 802.16a IEEE Standard, typically perform multi-carrier OFDM symbol encoding (which may include error correction encoding and interleaving), convert the encoded symbols into the time domain using Inverse Fast Fourier Transform (IFFT) techniques, and perform digital to analog conversion and conventional radio frequency (RF) upconversion on the signals. These transmitters then transmit the modulated and upconverted signals after appropriate power amplification to one or more receivers, resulting in a relatively high-speed time domain signal with a high peak-to-average ratio (PAPR).
Likewise, the receivers used in the wireless communication systems that are compliant with the aforementioned 802.11a/802.11g/802.11n and 802.16a IEEE standards typically include an RF receiving unit that performs RF downconversion and filtering of the received signals (which may be performed in one or more stages), and a baseband processor unit that processes the OFDM encoded symbols bearing the data of interest. The digital form of each OFDM symbol presented in the frequency domain is recovered after baseband downconverting, conventional analog to digital conversion and Fast Fourier Transformation of the received time domain signal.
To further increase the number of signals which may be propagated in the communication system and/or to compensate for deleterious effects associated with the various propagation paths, and to thereby improve transmission performance, it is known in the art to use multiple transmission and receive antennas within a wireless transmission system. Such a system is commonly referred to as a multiple-input, multiple-output (MIMO) wireless transmission system and is specifically provided for within the 802.11n IEEE Standard now adopted and being adopted in IEEE 802.16m and 3GPP-LTE Advance. As is known, the use of MIMO technology produces significant increases in spectral efficiency, throughput and link reliability, and these benefits generally increase as the number of transmission and receive antennas within the MIMO system increases.
In particular, in addition to the frequency channels created by the use of OFDM, a MIMO channel formed by the various transmissions and receive antennas between a particular transmitter and a particular receiver includes a number of independent spatial channels. As is known, a wireless MIMO communication system can provide improved performance (e.g., increased transmission capacity) by utilizing the additional dimensionalities created by these spatial channels for the transmission of additional data. Of course, the spatial channels of a wideband MIMO system may experience different channel conditions (e.g., different fading and multi-path effects) across the overall system bandwidth and may therefore achieve different signal-to-noise ratio (SNRs) at different frequencies (i.e., at the different OFDM frequency sub-bands) of the overall system bandwidth. Consequently, the number of information bits per modulation symbol (i.e., the data rate) that may be transmitted using the different frequency sub-bands of each spatial channel for a particular level of performance may differ from frequency sub-band to frequency sub-band.
In the MIMO-OFDM communication system, a high Peak-to-Average Power Ratio (PAPR) may be caused by the multiple carrier modulation. That is, because data are transmitted using multiple carriers in the MIMO-OFDM scheme, the final OFDM signals have amplitude obtained by summing up amplitudes of each carrier. The high PAPR results when the carrier signal phases are added constructively (zero phase difference) or destructively ('180 phase difference). Notably, OFDM signals have a higher peak-to-average power ratio (PAPR) than single-carrier signals do. The reason is that in the time domain, a multicarrier signal is the sum of many narrowband signals. At some instances in time, this sum is large and at other times is small, which means that the peak value of the signal is substantially larger than the average value. Similarly, MIMO schemes can have high PAPR for periodic sequence or binary-valued sequence.
High PAPR also results in MIMO schemes when multiple aggregate waveforms are transmitted in the same channel. In this instance, an aggregate waveform consists of a multiple combined individual waveforms/streams. The PAPR of an aggregate waveform is computed after combining the individual streams. Consequently, the highest PAPR amongst multiple aggregate waveforms is computed from the PAPRs of the combined individual waveforms. The continually increasing reliance on SISO and new focus on MISO/MIMO wireless forms of communication create an increasing need to decrease PAPR in these schemes.
Consider two similar channels, each with average power P0 and maximum instantaneous power P1. This corresponds to a peak-to-average power ratio PAPR=P1/P0, usually expressed in dB as PAPR[dB]=10 log(P1/P0). For the combined signal, the average power is 2 P0 (an increase of 3 dB), but the maximum instantaneous power can be as high as 4 P1, an increase of 6 dB. Thus, PAPR for the combined signal can increase by as much as 3 dB and, in general, the PAPR increases by 10 log(n) for n signal. This maximum power will occur if the signals from the two channels happen to have peaks which are in phase. This may be a rare transient occurrence, but in general the linear dynamic range of all transmitter components must be designed for this possibility. Nonlinearities will create intermodulation products, which will degrade the signal and cause it to spread into undesirable regions of the spectrum. When the signal nonlinearity is not negligible, the signal passed through the nonlinear device can be decomposed into a linear part and an uncorrelated distortion part, by utilizing Bussgang Theorem in disclosed,” “Bussgang, J. J., Cross-correlation functions of amplitude distorted Gaussian signals,” res. Lab. Elec., MIT, 1952. This, in turn, may require filtering, and in any case will likely reduce the power efficiency of the system. By negligible, what is meant is meant is the value under consideration is within the tolerance required for that value within that system. A negligible value may have minimal impact on the receiver function.
This problem of the peak-to-average power ratio (PAPR) is a well-known general problem in OFDM and related waveforms, since they are constructed of multiple closely-spaced sub-channels. There are a number of classic strategies to reducing the PAPR, which are addressed in such review articles as “Directions and Recent Advances in PAPR Reduction Methods”, Hanna Bogucka, Proc. 2006 IEEE International Symposium on Signal Processing and Information Technology, pp. 821-827, incorporated herein by reference. These PAPR reduction strategies include amplitude clipping and filtering, coding, tone reservation, tone injection, active constellation extension, and multiple signal representation techniques such as partial transmit sequence (PTS), selective mapping (SLM), and interleaving. These techniques can achieve significant PAPR reduction, but at the expense of transmit signal power increase, bit error rate (BER) increase, data rate loss, increase in computational complexity, and so on. EVM is the most popular figure of merit for evaluating in-band distortions in a communication system. Compared with BER, EVM not only quantifies the distortions, but also attributes the source of distortion to phase noise, modulator imbalances, power amplifier nonlinearities, and so on. EVM is more useful than BER when performing real-world RF design and troubleshooting. Because of its excellent troubleshooting capability, EVM has been adopted by many communication standards, such as wideband CDMA, wireless LAN, and wireless MAN. EVM has also been taken into account when formulating CFR algorithms. Further, many of these techniques require the transmission of additional side-information (about the signal transformation) together with the signal itself, in order that the received signal to be properly decoded. Such side-information reduces the generality of the technique, particularly for a technology where one would like simple mobile receivers to receive signals from a variety of base station transmitters. To the extent compatible, the techniques disclosed in Bogucka, and otherwise known in the art, can be used in conjunction with the techniques discussed herein-below.
Various efforts to solve the PAPR (Peak to Average Power Ratio) issue in an OFDM transmission scheme, include a frequency domain interleaving method, a clipping filtering method (See, for example, X. Li and L. J. Cimini, “Effects of Clipping and Filtering on the Performance of OFDM”, IEEE Commun. Lett., Vol. 2, No. 5, pp. 131-133, May, 1998), a partial transmit sequence (PTS) method (See, for example, L. J Cimini and N. R. Sollenberger, “Peak-to-Average Power Ratio Reduction of an OFDM Signal Using Partial Transmit Sequences”, IEEE Commun. Lett., Vol. 4, No. 3, pp. 86-88, March, 2000), and a cyclic shift sequence (CSS) method (See, for example, G. Hill and M. Faulkner, “Cyclic Shifting and Time Inversion of Partial Transmit Sequences to Reduce the Peak-to-Average Ratio in OFDM”, PIM RC 2000, Vol. 2, pp. 1256-1259, September 2000). In addition, to improve the receiving characteristic in OFDM transmission when a non-linear transmission amplifier is used, a PTS method using a minimum clipping power loss scheme (MCPLS) is proposed to minimize the power loss clipped by a transmission amplifier (See, for example, Xia Lei, Youxi Tang, Shaoqian Li, “A Minimum Clipping Power Loss Scheme for Mitigating the Clipping Noise in OFDM”, GLOBECOM 2003, IEEE, Vol. 1, pp. 6-9, December 2003). The MCPLS is also applicable to a cyclic shifting sequence (CSS) method.
In base station towers today, most 3G operators combine several narrow band signals and transmit them through a common power amplifier signal. However, the signal characteristics for 3G networks (such as WCDMA) differ greatly from current 4G technologies such as OFDM and OFDMA technologies, which tend to have extreme maxima and minima in their signal envelope, compared to nearly constant signal envelope. Spectrum combination for wide bandwidths is inherently more challenging for amplifier designers in terms of maintaining linearity across the total band. In addition, very high PAPR exacerbate the poor operational efficiency for the wideband power amplifier (PA). These PAs with lower efficiencies result in greater heat dissipation requiring better heat transfer mechanisms which lead to larger base stations, increasing operator's capital and operating expenditure. On the contrary, consider the case of over-driving the PA. As the amplifier over-drive is increased, higher power efficiency is achieved. As an example—disclosed in S.-C. Jung, O. Hammi, and F. M. Ghannouchi, “Design optimization and DPD linearization of GaN-based unsymmetrical Doherty power amplifiers for 3G multicarrier applications,” IEEE Microw. Theory Techn., vol. 57, no. 9, pp. 2105-2113, 2009.—when a linearized Doherty amplifier is over-driven by 3 dB, its efficiency increases to 47.52%, in contrast to the case of 38.03% efficiency when no over-drive is applied. This shows around 9% increase in PA's power efficiency with 3 dB over drive. However, in order to not impact the performance other active users of the spectrum, over-drive one's PA even beyond the limit imposed by the spectrum criteria of the adjacent channels where the spectrum limit is not met must be idle. What is need is an adaptive scheme which maximum amplifier efficiency while controlling the out-of-band interference to other users.
As noted earlier, many PAPR reduction techniques lead to distortion of the transmit signal characteristics, which is quantifiable by Error-Vector-Magnitude (EVM) and out-of-band spectral emissions. A goal of this invention is to minimize the effect of distortion on EVM and out-of-band spectral emission while reducing PAPR, which allows use of higher order modulation scheme to result in higher spectral efficiencies for the network. The error vector magnitude is a measure used to quantify the performance of a digital radio transmitter or receiver. A signal sent by an ideal transmitter or received by a receiver would have all constellation points precisely at the ideal locations, however various imperfections in the implementation (such as carrier leakage, low image rejection ratio, phase noise etc.) cause the actual constellation points to deviate from the ideal locations, EVM is a measure of how far the points are from the ideal locations. An error vector is a vector in the real and imaginary plane between the ideal constellation point and the point received by the receiver. In other words, the error vector may be the difference between actual received symbols and ideal symbols. The average power of the error vector, normalized to signal power, is the EVM. For the percentage format, root mean square (RMS) average is used. For example, the RMS may be, the square root of the arithmetic mean of the squares of the signal amplitudes, or the square of the function that defines the continuous waveform. In comparison, out-of-band spectral emission is predicated on the definition of what is considered the occupied bandwidth of the incoming signal. One common definition used in the art is the 99% occupied bandwidth, which is defined as the bandwidth that contains 99% of the total power of the signal. Governing regulatory body of the countries define a spectrum mask of allowable power relative to the total power that a device may transmitter in specified frequency bandwidths at certain offsets from the center of the occupied bandwidth. The out-of-band spectral emission is defined as the accumulative measured power starting at a frequency offset of 0.5 times the occupied or necessary bandwidth and extends up to 2.5 times the occupied or necessary bandwidth, respectively. In the present invention, the fitness function is constrained by the allowable over-drive level while meeting the linearity mask, which is adaptively estimated by an adjacent channel leakage ratio (ACLR) sensing algorithm, incorporated in the transmitter, that utilizes input drive signal power to the PA.
The continually increasing reliance on especially MISO wireless forms of communication creates a need for means to reduce the PAPR, especially in multi-carrier, multi-dimensional systems.
Then according to the prior art, what is needed is a system and method that reduces the PAPR of a data transmission by eliminating the guesswork involved in randomly generating indexed samples for PAPR optimization.